Qualitative and quantitative analyses of Coleus forskohlii , “an ingredient calling for special attention” in a health food

Abstract

In 2020, the government of Japan has designated Coleus forskohlii as “an ingredient calling for special attention”. To ensure the efficacy and safety of health food products containing C. forskohlii, it is essential to identify and quantify the active compounds present in that plant. In this study, we established a specific TLC identification method and a quantitative HPLC assay, and evaluated the quality and quantity of 14 health food products. For TLC identification and for a cleaner analysis method, we modified the USP method in which toluene is used as the TLC eluent. Forskolin and its isomer, isoforskolin, were clearly separated with n-hexane/ethyl acetate (1:1) and detected with 4-methoxybenzaldehyde/sulfuric acid/ethanol (1:1:18) spray on TLC plates. For HPLC quantification of forskolin in C. forskohlii-containing products, we developed a simpler and more sensitive method based on the USP monograph. Good linearity with a coefficient of determination (r² > 0.998) was achieved over a concentration range 2–4000 μg/mL. The limit of detection and the limit of quantification were estimated to be 0.42 and 1.28 μg/mL, respectively. The precision of the method was confirmed by 92.01% forskolin recovery. Surprisingly, the forskolin content in health food products varied greatly and ranged up to 300 fold (0.35–120.10 mg/day). Thus, it was considered that the differences in the forskolin content in the sample products were caused by the differences in their merchandise design. These developed methods would be useful for the qualitative and quantitative evaluations of health food products containing C. forskohlii.